It is known in the art to provide an identification system using transponders communicating with an identification receiver. For example, U.S. Pat. No. 5,491,468, issued to Everett et al., discloses a portable tag which receives energy from a reading device via magnetic coupling for charging a storage capacitor. A discharge of the capacitor powers a coded information transmission circuit during a small percentage of the duty cycle. Transmissions are made from the portable tag to the reading device.
It is also known in the art to provide an identification system using transponders communicating with an identification receiver to reduce the probability that more than one transponder simultaneously transmits to the receiver at a same frequency. U.S. Pat. No. 5,302,954, issued to Brooks et al., and U.S. Pat. No. 5,153,583, issued to Murdoch, disclose a base station for applying a magnetic field to a plurality of transponders. Each of the transponders extracts energy from the magnetic field. The energy extracted by individual ones of the transponders enables the individual transponders to transmit identification codes and/or specially stored or other information to be identified by a base station receiver.
The transponders can generate one or more carrier frequencies from an available set of carrier frequencies. As such, many transponders simultaneously transmitting to the base station may be identified under conditions where co-interference would normally preclude correct identification. An idle state, during which individual ones of the transponders do not transmit signals, is employed to reduce the probability that more than one transponder will transmit signals at the same frequency, thereby ensuring that correct identification of a transmitting transponder is made. Signals which may have been corrupted or co-interfered with can be ignored by the receiver. Each transponder can sequentially transmit an identifying code on a randomly selected frequency that is selected from an available set of carrier frequencies.
The use of an idle state and randomly selected frequencies may reduce the probability that more than one transponder will transmit signals of a same frequency at a same time. However, the degree of reduction attainable by these techniques is still limited because, for example, there are typically a restricted number of frequency bands available owing to finite receiver and/or transmitter bandwidths.